Patent GB-818147-A (GB)
Title: Electrical measuring apparatus incorporating automatic range selection
IPC: G01R15/09
Filed: 1956-03-21
Abstract: 818,147. Voltage and resistance measurement; measuring bridges. DUBILIER, W., DUBILIER, F:, DUBILIER, W. Jun., and DUBILIER, M. H. [trading as RADIO PATENTS CO.]. March 21, 1956 [March 28, 1955; Jan. 18, 1956], No. 8896/56. Class 37. [Also in Group XXXVIII] A voltage measuring device comprises an indicator constructed and operating similarly to a valve voltmeter, an automatic range selector switch and two electronic switching devices, each connected to a distinctive output electrode of the voltmeter and respectively operative in response to out of balance voltages on the electrodes due to a measurement input voltage, a relay being provided to be actuated in response to such switching devices to energize a drive mechanism or servomotor enclosure of a circuit, whereby the range selector switch is driven by the motor to adjust a range multiplier in the voltmeter valve input through digital steps so long as the outof-balance voltages are greater (or less) than a predetermined value. Structure.-Fig. 4 shows an automatic range switching voltage measuring device wherein the input is connected across a 6-position tapped range selecting potentiometer 17a-17f whose slider 33 is connected to the grid of tube 14a of a vacuum tube voltmeter the grid of whose other tube 14b is connected to the slider 48 of an equivalent tapped balancing potentiometer 47a-47f. Meter 15 in series with calibration resistance 30 is connected across the cathodes in parallel with zero potentiometer 31 centre-tapped to earth, while the respective anodes are resistance coupled to the grids of thyratrons 21a, 21b through respective voltage setting potentiometers 24a, 24b; the thyratron anodes being co-phasedly supplied with A.C. and interconnected by relay 22 in series with normally open contacts 42 of operating pressel switch 38 whose remaining normally open contacts 43 are connected across the voltmeter tube cathodes. Contact 23 closes on energization of relay 22 to energize a D.C. motor E geared to adjust the sliders of 6 position tapped potentiometers 17 and 47 together with 6 position switch 25a to 25f arranged to shortcircuit respective resistances 56, 57 at opposite ends of the range of potentiometer adjustment. The motor also drives contacts 50, 150 to traverse 12 position switches 16a-16l, 116a- 116l (the latter contacts occupying 360 degrees of rotation while the contacts 17, 47 occupy 180 degrees and are each duplicated with a common zero in two interconnected banks, Fig. 7) connected together in banks 16a-16f, 16g-16l, 116a-116f and 116g-116l and arranged to interchange the input and balance potentiometers between the grids of triodes 14a, 14b when sliders 33, 48 are driven through the zero position. The servomotor may be of the continuously rotating or stepping switchtype. Operation.-An unknown voltage at the input with switches 42, 43 open produces a deflection of the valve voltmeter in dependence on its magnitude and the position of potentiometer 17a-17f, which may be below or above the indicating part of the scale giving highest accuracy. Closure of switches 42, 43 disable the voltmeter and complete the circuit of relay 22. The voltmeter anodes produce opposed voltages of which that at 14b increases with rising input and that at 14a increases with falling input, so that as the input increases above a level set by potentiometer 24a, thyratron 21a fires, while as it decreases below a level set by potentiometer 24b, thyratron 21b fires. For a voltage exceeding the voltmeter range, relay 22 thus operates and motor E drives the sliding contacts of the potentiometers 17, 47 over the contacts in the sense a to f until a range is selected at which the unknown voltage is on the appropriate part of the meter scale, and is measurable on release of switches 42, 43 with reference to multiple scales of the meter (Figs. 5, 6, not shown). Variations of input voltage carrying the indications off the voltmeter scale may be accommodated by closure of switches 42, 43 and operation of the apparatus to select new measuring ranges. When the highest range is selected, contact 25a of six-position switch 25a to 25f driven by the motor shortcircuits resistance 57 in potentiometer 24b at the grid of thyratron 21b, which is biased to non-conductivity however high the voltage applied from tube 14b. Similarly, when the lowest range is selected, contact 25f shortcircuits resistance 56 in the potentiometer 24a at the grid of thyratron 21a so that it is biased to non-conductivity at any voltage greater than zero. The occurrence of a negative input voltage will, however, fire thyratron 21a and start the motor to drive the potentiometer contacts through zero to select successively increasing ranges in the opposite sense; the polarity reversing switches operating at zero to connect potentiometer 17 in the grid of triode 14b and potentiometer 47 in the grid of triode 14a. General.-In modifications (Fig. 8, not shown) resistance ohms are measurable by connecting a battery in series with the unknown and a range of series resistances selectible by a servo system to produce valve voltmeter readings of ohms within a certain range which are multiplied by a constant appropriate to the selected series resistance; and alternating voltage is measurable by adding a rectifier ahead of the valve voltmeter input potentiometer chain; the instrument then indicating on a single polarity. An independent potentiometer chain may be provided for A.C.. measurement. In a further modification (Fig. 9), for use as a servomechanism responsive to a D.C. energized potentiometer 98 adjusted by an input shaft 95, the output thereof is applied to the input potentiometer 10 of the voltage measuring apparatus described; in which the valve voltmeter cathodes are permanently interconnected and the limit and polarity reversing switches are omitted. In operation, motor E readjusts potentiometer 10 for balance and rotates output shaft 33 to represent the input to a degree of accuracy determined by the intervals between the tapping contacts. The valve voltmeter may be operative to give vernier indications between adjacent tappings, and a single balanced measuring device may be switchably connectible in sequence to successive voltages to be indicated.